Ventilatory inefficiency during graded exercise in COPD: A pragmatic approach

Abstract

Background/objective: The current approach to measuring ventilatory (in)efficiency (V'_E -V'CO₂ slope, nadir and intercept) presents critical drawbacks in the evaluation of COPD subjects, owing mainly to mechanical ventilatory constraints. Thus, we aimed to compare the current approach with a new method we have developed for ventilatory efficiency calculation.

Methods: The new procedure was based on measuring the amount of CO₂ cleared by the lungs (V'CO₂ , L/min) plotted against a predefined range of increase in minute ventilation (V'_E ) (ten-fold increase based on semilog scale) during incremental exercise to symptom-limited maximum tolerance. This value was compared to a hypothetical predicted maximum CO₂ output at the predicted maximal voluntary ventilation, defining ventilatory efficiency (ηV'_E , %). The results were used to compare 30 subjects with COPD (II-IV Global Initiative for Chronic Obstructive Lung Disease, GOLD) and 10 non-COPD smokers, to establish the best discriminative physiological variable for disease severity through logistic multinomial regression.

Results: The new approach was more sensitive to progressive deterioration of airway obstruction, resulting in worse ηV'_E as lung function worsens throughout the GOLD panel (ηV'_E (%), p < .001), when compared with V'_E -V'CO₂ slope (p = .715) or V'_E -V'CO₂ nadir (p = .070), besides showing the best model based on the logistic regression approach.

Conclusion: Although requiring more complex calculations compared to the current procedure, the new approach is highly sensitive to true ventilatory/gas-exchange deterioration, even throughout more severe pulmonary lung function in COPD subjects.